Vary wire length while controlling material, cross-sectional area and temperature. Measure current and p.d. for each length and calculate R = V/I. For a uniform wire at constant temperature, resistance increases in direct proportion to length; disconnect between readings to limit heating. Example: Interpret lengths 0.2 m and 0.4 m with resistances 2 Ω and 4 Ω. Doubling length doubles resistance; these data support proportionality if the other variables are controlled. Distinguish the quantities.